1. Field of the Invention
The present invention is directed to a resonator suitable for use in a magnetic resonance imaging apparatus (MRI or nuclear magnetic resonance (NMR) for diagnostic examination of an examination subject, such as a human body.
2. Description of the Prior Art
In a magnetic resonance imaging apparatus which is used for medical diagnostic of a human body, the body axis is usually oriented along the X-axis of a rectangular coordinate system. The body region to be examined is situated between the pole pieces of a magnet which generates a fundamental magnetic field extending in the direction of the z-axis. A resonator is provided for transmitting the excitation signals and receiving the resonance signals. It is known to construct such a resonator as a combination of two sub-systems, which are respectively connected to a transmitter and a receiver via a network which serves the purpose of frequency tuning, load matching and balancing and via a common 90.degree./3dB directional coupler.
In addition to obtaining tomograms of an examination subject, magnetic resonance imaging can be used to undertake diagnostics of joints and to portray blood vessels. An image is constructed by computational or mensurational analysis of integral proton resonance signals from the spatial spin density, or analysis of the of the distribution of relaxation times, of the examination subject. The examination subject is introduced into a uniform magnetic field, referred to as the fundamental field, which aligns the nuclear spins in the body. Gradient coils are provided which generate spatially different magnetic fields. A RF antenna excites the nuclear spins, and receives the measured signals induced by the excited nuclear spins, which are forwarded to a receiver. This RF antenna is generally connected to a transmitter and a receiver via a network having matching capacitances, as well as via a transmission and reception diplexer. The maximum pulse transmission power is established by the load limit of these components, whereas the maximum mean transmission power is essentially limited by the extent to which localized heating of the examination subject can be safely tolerated.
As is known, a low transmission power requires circularly polarizing antennas. Such antennas have the advantage of generating only the field components which are effective for the nuclear magnetic resonance, for example counter-clockwise field components. Such an antenna, for example, may be formed by two linearly polarizing antenna systems arranged orthogonally relative to each other, and connected to a transmitter and to a receiver via a 90.degree. directional coupler. The supplied transmission signal is divided between the two systems with a 90.degree. phase shift, and generates the rotational field which is effective for the nuclear magnetic tomography. In the reception mode, the antenna represents two useful signal sources phase-shifted by 90.degree., and also represents two uncorrelated noise sources. The 90.degree. directional coupler supplies the receiver with the in-phase sum of the useful signals. Such as antenna system is described in the article "Quadrature Detection Coils- A Further .sqroot.2 Improvement in Sensitivity," Chen et al., Journal of Magnetic Resonance, Vol. 54, (1983) pp: 324-327.
Antenna systems known as surface resonators are also known which can be used for obtaining an image of certain body regions having relatively small extent. Such known surface resonators are flat ("pancake") coils having one or more turns. Such surface resonators are simply placed on the body part for which an image is to be obtained, for example, on a spinal vertebra, the middle ear, or an eye.
A known, circularly polarizing surface resonator for magnetic resonance imaging of a human body, whose body axis extends in the direction of the fundamental magnetic field, consists of two nested sub-systems. One of the sub-systems, known as a planar pair resonator, contains two annular-cylindrical coil turns formed by ribbon conductors. These coil turns are connected to each other via other conductors. The two coil turns are arranged side-by-side in the x-z plane. The second sub-system is known as a CRC (counter rotating current) resonator, and also contains two annular-cylindrical coil turns which are arranged above each other co-axially relative to the y-axis and parallel to the x-z plane. The planar pair resonator is disposed in the space between the two coils of the CRC resonator. An intrinic decoupling from uniform, external high frequency fields is obtained with this resonator. This embodiment has two differently constructed and arranged sub-systems, however, and is therefore suitable only for use with an apparatus wherein the fundamental magnetic field proceeds in the direction of the body axis, and moreover is relatively complicated. A system of this type is described in the article "Quadrature Detection Surface Coil," Hyde et al., Magnetic Resonance in Medicine, Vol. 4(1987), pp. 179-184.
Known fundamental field magnets are generally superconductive for stronger magnetic fields above 0.5T, and are generally in the form of solenoids, which generate a static fundamental field proceeding in the direction of the body axis of the patient. Other fundamental field magnets are also known for magnetic resonance imaging wherein the fundamental field extends perpendicular to the body axis of the examination subject, i.e., in the direction of the z-axis of a rectangular coordinate system. The magnet for generating such a fundamental magnetic field has pole pieces which define an imaging region, and between which the uniform fundamental field is generated. The pole pieces may be connected to each other via the yoke of a permanent magnet or of an electromagnet, and may form a C-magnet or H-magnet having two yokes, as described in European application 0 161 782. The fundamental magnetic field generated by such magnets is known as a transversal fundamental field.